linux-kernel - Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

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Date:	Sun, 24 Aug 2008 11:01:33 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

> > Ah so it was already 25% slower even without kmalloc? I thought
> > that was with already. That doesn't sound good. Any idea where that slowdown 
> > comes from?
> 
> Just longer code path, I think.  It calls the generic

I did IPI measurements quite some time ago and what I remember
from them is that IPI latencies were in the low multiple thousands cycle
ballpark.

> smp_call_function_mask(), which then does a popcount on the cpu mask
> (which it needs to do anyway), sees only one bit set, and then punts to
> the smp_call_function_single() path.  

But that is more in the a few tens of cycles (or maybe 1-2 hundreds
if you have a NR_CPU==4096 kernel with really large cpumask)

Doesn't really explain a 25% slowdown I would say. 

Are you sure there isn't a new cache miss in there or something? Actually
it must be even multiple ones to account for such a slow down.

-Andi

-- 
ak@...ux.intel.com
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